In our first game we did have two wandering monsters turn up, but they were so far away from the action that the never really took part in the game.
We were thinking that, as you test for monsters when someone picks up a treasure, the beastie has been disturbed/woken up/angered by that action. Roll a random direction from the treasure and the monster emerges from the first piece of cover where it has been hiding/sleeping. With the proviso that it must be at least 6" from any character, except for the idiot who has disturbed it!
How do you more experienced guys think that would work?